Assessment Comments

Assessment is based on results of (1) IDNR/UHL monthly ambient water quality monitoring downstream from Sac City from 2000-02 and evaluated IDNR/UHL biological (biocriteria) monitoring conducted in 2001.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality standards.   The Class B(WW) aquatic life uses are assessed (evaluated) as "partially supporting" based on results of IDNR/UHL biological (biocriteria) monitoring in 2001.   The fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) the results from the IDNR/UHL ambient monthly monitoring station downstream from Sac City in Sac County (STORET station 10810001 (formerly station 423014)) located approximately 5 miles south of Sac City from 2000-02 and results of IDNR/UHL biological (biocriteria) monitoring in 2001.

EXPLANATION:  The Class A uses are assessed as "not supported" due to levels of indicator bacteria (fecal coliforms) that exceed state water quality criteria.   For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff.   According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff."  Twenty-four of the 34 samples collected from the Sac City station during the 2000, 2001, and 2002 recreational seasons were collected at flows not materially affected by surface runoff.   The geometric mean level of indicator bacteria (fecal coliforms) in these 24 non-runoff-affected samples (215 orgs/100ml) is above the Iowa Class A water quality criterion of 200 orgs/100ml.   Seven  four of the 24 samples (30%) exceeded the U.S.  EPA-recommended single-sample maximum value of 400 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting, geometric means for fecal coliforms that exceed 200 organisms/100 ml indicate “nonsupport” of primary contact recreation uses (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).  

The Class B(WW) aquatic life uses were also assessed (evaluated) using biological data collected in 2001 as part of the ambient water monitoring project and the DNR/UHL stream biocriteria project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2001 evaluated Fish IBI score was 54 (good) and 2001 evaluated BM-IBI score was 51 (fair), respectively.  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   The reason this site was evaluated instead of monitored for this cycle was because of the size of the waterbody.   The FIBI and BMIBI criteria were based and calibrated on wadeable streams and rivers.   This waterbody is on the upper end of being classified as a wadeable site; therefore, the criteria used to assess this may not accurately reflect the condition of the biological community.

Results of IDNR/UHL ambient chemical/physical water quality monitoring at the Sac City station during the 2000-2002 assessment period, however, suggest relatively good water quality in this river segment.   Results of this monitoring show no violations of Class B(WW) (aquatic life) water quality criteria in the 52 samples analyzed for dissolved oxygen and pH, in the four samples analyzed for toxic metals, or in the 10 samples analyzed for pesticides and other toxic organic compounds.   The sample collected on March 19, 2001, however, contained 4.1 mg/l of ammonia-nitrogen and thus violated the Class B(WW) chronic water quality criterion.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), this one violation of a water quality criterion for a toxic parameter in an "abundant" data set does not suggest an impairment of the aquatic life uses.   Based on IDNR's assessment methodology for Section 305(b) reporting, however, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."  Also, one of the 19 samples analyzed for pesticides contained the pesticide chlorpyrifos at a level that exceeded the state water quality criterion.   The sample collected on November 27, 2001, contained 0.78 ppb of chlorpyrifos; this level exceeds the Class B(WW) chronic criterion of 0.041 ppb.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), this single violation of chronic criteria chlorpyrifos does not suggest an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, however, this violation does suggest that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."

Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.   The most recent fish contaminant monitoring was conducted in 1993 as part of the U.S.  EPA/IDNR RAFT program.   These data are too old to accurately characterize current contaminant levels.